The present invention relates to a discharge electrode for use in a charger which is incorporated in an electrophotographic image forming apparatus, electric dust collector, sewage treating apparatus or similar apparatus and, more particularly, to a thin wire included in such a charger to serve as a discharge electrode.
Generally, an electrophotographic image forming apparatus such as an electrophotographic copier has a photoconductive element having a photoconductive layer on the surface thereof. The photoconductive element is uniformly charged to a predetermined polarity by a charger having a discharge electrode. The discharge electrode has customarily been implemented as a thin wire of tungsten, stainless steel or similar wire and oxidized or plated with gold. The oxidation forms an oxide layer for preventing an oxide film from being formed on the surface of the thin wire in the event of discharge. This kind of discharge electrode effects a corona discharge when applied with a voltage of 4 kV to 7 kV.
It is likely with the thin wire of the above-stated discharge electrode that its surface suffers from regeneration and deterioration due to aging and, in due course, fails to achieve a uniform charging or discharging characteristic in the axial direction of the electrode. Such an occurrence is ascribable mainly to the fact that the uniform surface condition of the thin wire is disturbed by the corrosion of the surface of the wire due to repeated discharge and by the crack or separation of the oxide layer or that of the plated layer. Another major cause is the deposition of oxides and ionization products on the thin wire due to the ionization of air components, moisture, ozone and various impurities such as dust particles exiting between discharge electrodes which is brought about by discharge energy in the event of discharge.
In the light of the above, a discharge electrode having a thin wire made of amorphous alloy has been proposed. Specifically, an aluminum trioxide (Al.sub.2 O.sub.3) film may be formed on the surface of a thin film of Fe--Si--B amorphous alloy to a thickness of 5000 .ANG. by sputtering, as disclosed in Japanese Patent Laid-Open Publication (Kokai) No. 132966/1986. Another approach is to form the entire thin wire by use of amorphous alloy, as also proposed in the art.
A discharge electrode entirely made of amorphous alloy as mentioned above little suffers from regeneration and deterioration on the surface thereof, but providing such a discharge electrode with an outside diameter as small as several ten microns in the amorphous state and in uniform dimensions in both of the sectional and longitudinal directions would be extremely difficult, if not possible, and would need a disproportionate production cost.
Assume that the amorphous alloy for coating the surface of the thin wire contains 12 at % (atomic percent) of tungsten (W) and is deposited on the thin wire to a thickness of 0.5 .mu.m. A drawback with this kind of discharge electrode is that as it is repeatedly used, a white product whose major component is siliocon dioxide (SiO.sub.2) sometimes deposits on the electrode surface in a needle-like configuration. Such a white product is apt to effect the uniform discharge current distribution in the axial direction of the thin wire. Presumably, the deposition of the white product is caused by silicon oil which is used in a fixing device of an electrophotogaphic copier for the separation of a toner and is evaporated by heat to produce silicon. This phenomenon is especially conspicuous when use is made of a thin wire containing tungsten (W). Since the above-mentioned deposit is insulative as apparent from the component, the discharge current noticeably fluctuates in the axial direction of the thin wire and, hence, it is impossible to set up a uniform charging or discharging condition over the entire discharging range.